Auxin is an important signal chemical that controls plant growth and development. It controls plant cell division, expansion and differentiation, as well as lateral root formation and flowering. (Davies P. J. (2004) The Plant Hormone: Their Nature, Kluwer, Dordrecht, the Netherlands). The biosynthetic pathway of auxin at the genetic level has remained unclear. Two major pathways have been proposed: The tryptophan(Try)-independent and Trp-dependent pathways. See, Mashiguchi et al. (2011) PNAS 108:18512-18517). There are also auxin synthase genes in microorganisms (Thomashow et al. (1984) PNAS 81:5071-5075; Van Onckelen et al. (1986) FEBS Lett 198:357-360. Although the research on auxin is very extensive, the synthesis pathways and the function of auxin remain unclear in plants.
In recent years, the auxin synthase gene has been used as a tool for crop improvement. The auxin synthase gene has been expressed in the epithelial cells of cotton ovule resulting in transgenic cotton with traits of high quality and high yield. (Zhang et al. (2011) Nature Biotechnology 29: 453-458). The auxin synthase gene was also expressed in grape driven by the ovule-specific promoter resulting in the increase in grape fecundity. (Costantini et al. (2007) Plant Physiol 143:1689-1694). However, there are still no effective methods to improve the yield of major crops, such as rice, corn, soybean, wheat, barley, sorghum and sunflower. The impact of auxin on plants is complicated. In fact, excess auxin can be highly harmful to plant. Therefore, methods are needed to utilize auxin to enhance crop yield.